1. Field of the Invention
The present invention relates to a tank cap to close a tank opening member and a tank cap apparatus with the tank cap attached thereto.
2. Description of Related Art
In conventional practice, tank caps are configured such that a fuel supply port is closed by rotating a fuel cap fitted with a gasket several times relative to a filler neck connected to a fuel tank. Since rotating the fuel cap a plurality of times sometimes fails to result in a tight fit, closing the fuel supply port of a filler neck with a fuel cap merely by turning the cap through a predetermined angle (for example, about 90°) has been proposed as a way of overcoming this shortcoming.
FIG. 24 is a diagram depicting the state existing before a fuel cap is mounted over a filler neck. The fuel cap 100 comprises a casing main body 102 for opening and closing the fuel supply inlet FNb of a filler neck FN, a cover 104 mounted on the casing main body 102, and a gasket GS mounted in the upper portion of the casing main body 102. A ratchet mechanism (not shown) is interposed between the casing main body 102 and the cover 104 to ensure that the cover 104 idles relative to the casing main body 102 when excessive torque is applied between the cover 104 and the casing main body 102.
A casing engagement element 102a is also formed in the lower portion of the outer circumference of the casing main body 102. An opening engagement element FNc is further formed in the inner circumferential portion of the filler neck FN. Part of the area around the inside of the opening engagement element FNc is provided with a neck insertion notch FNd for allowing the casing engagement element 102a of the fuel cap 100 to be inserted in the axial direction.
FIG. 25 is a diagram depicting the manner in which the fuel cap 100 is mounted over the filler neck FN. The opening engagement element FNc is tapered by a prescribed inclination α in the axial direction, and the guide surface 102b of the casing engagement element 102a is tapered to match this angle.
The manner in which the inlet FNb of FN is closed with the fuel cap 100 will now be described. The casing engagement element 102a is positioned in the neck insertion slot FNd, and the fuel cap 100 is turned through a predetermined angle (about 90°) while the fuel cap 100 is inserted into the filler neck FN. By this, the fuel cap 100 is mounted over the filler neck FN as a result of the fact that the casing engagement element 102a is caused to align with and engage the opening engagement element FNc. At the same time, the gasket GS is compressed between the casing main body 102 and the filler neck FN, forming a seal. If rotated further, the cover 104 will idle due to the presence of the ratchet mechanism, signaling that the limit has been reached and indicating that the fuel cap 100 fits tightly over the inlet FNb.
Reducing the inclination a of the opening engagement element FNc during the closure of the fuel cap 100 will increase the operating angle of the fuel cap 100, not only making the cap more difficult to operate but also allowing the fuel cap 100 to occasionally rotate through an angle greater than 180° and come off. Conversely, increasing the inclination α will increase the tightening rate of the gasket GS (that is, the extent to which the gasket GS is compressed) and will enhance the rubber recoil of the gasket GS. For this reason, the rotational torque will increase and the cover will be idled by the ratchet mechanism before the gasket GS is adequately compressed, making it impossible to form a tight seal.
This approach is thus disadvantageous in the sense that it is difficult to cause the gasket GS to form a tight seal by rotating the fuel cap 100 through a narrow operating angle (about 90°). In addition, considerable load is applied between the opening engagement element FNc and the corner 102c of the casing engagement element 102a when the corner 102c is pressed against the opening engagement element FNc. A greater rotational torque is therefore needed to close the fuel cap 100, sometimes making it impossible to close the cover properly and form a tight seal.